1. Field of the Invention
The present invention relates to a multichannel access type cordless telephone.
2. Description of the Related Art
Two control channels and 87 communication channels are available for low power type cordless telephones in Japan. When a call is placed or received, a control channel is first used to connect the base unit and cordless handset and, thereafter, communication can be started using a communication channel.
In the case of cordless telephones in the United States, no control channel is available and only a plurality of control channels are prepared like very low power type systems in Japan.
The FCC (Federal Communications Commission) in the United States first approved five channels for cordless telephones and another five channels were added later to total the approved channels at ten. Since the number of communication channels has become insufficient as a result of the widespread use of cordless telephones, it is expected that another fifteen channels will be added to provide twenty five channels in total.
In a cordless telephone for which no control channel is available as described above, the base unit and cordless handset are required to always monitor the communication channels in the standby mode so as to be able to respond to a connection request from each other (cordless handset or base unit) any time.
Increasing the number of communication channels to twenty five as mentioned above, however, will require considerable time for all of such communication channels to be monitored.
Switching of a communication channel to another is generally carried out by changing the dividing ratio of a variable frequency division circuit in a PLL (phase locked loop). Therefore, in the case of a microcomputer for system control, it takes about 25 ms for the PLL to be stabilized after the data of the new dividing ratio after a channel change is generated and loaded into the variable frequency division circuit of the PLL. Further, it takes about 35 ms before the presence or absence of an electric wave from the base unit or cordless handset is detected in any communication channel.
Therefore, it takes 1.5 s ((25 ms+35 ms).times.25 channels) to make a round of the entire communication channels. That is, such a long period of time is required for monitoring all of the communication channels only once.
As a result, it takes an undesirably long time for a cordless handset before inputs on the dial keys thereof are accepted after the call key is pushed when a call is placed to a public switched telephone network.
In addition, if a cordless handset always monitors the communication channels, the power consumption of the rechargeable battery which feeds the cordless handset will be accelerated. This shortens the operable period of the cordless handset supported by one charging operation.
Taking the above-described situation into consideration, a method has been proposed wherein a scan mode for sequentially monitoring all the communication channel once and a sleep mode (a mode wherein the power supplies of all circuits except the microcomputer are turned off for a predetermined period of time) are alternately enabled. This method makes it possible to suppress the consumption of the battery because even the microcomputer consumes almost no current in the sleep mode.
In this case, however, such an effect is reduced unless the period of the sleep mode is sufficiently longer than that of the scan mode. If the number of the communication channels is as large as twenty five as described above, it takes a considerably long time to complete one cycle of the combined scan and sleep modes. This is impractical because it takes a considerably long time before a request for reception of a call from the base unit is accepted by the cordless handset.
It is an object of the present invention to solve the above-described problems.